From rayner@galileo.ifa.hawaii.edu Fri Dec 3 10:02:50 1999 Date: Sun, 28 Nov 1999 13:51:16 -1000 (HST) From: John Rayner To: bergknut@jeans.ifa.hawaii.edu, denault@jeans.ifa.hawaii.edu, gching@galileo.ifa.hawaii.edu, kuwata@jeans.ifa.hawaii.edu, neill@galileo.ifa.hawaii.edu, onaka@jeans.ifa.hawaii.edu, rayner@galileo.ifa.hawaii.edu, toomey@jeans.ifa.hawaii.edu, vern@hokuhele.ifa.hawaii.edu, watanabe@jeans.ifa.hawaii.edu With the mods completed and the cryostat reassembled I repeated the flexure test of the slit wheel and grating turret. The good news is that the slit wheel movement is much improved but the bad news is that the grating turret movement is different and worse (12 pixels @ 60 degrees tilt). Immediate plan: Greg should remove the truss leg first thing Monday morning so that we can remove the grating turret as soon as I've shown Vern the play in the mechanism. Vern and Doug should discuss mods asap. (You may need to come over, Doug.) I'll then decide on what to do about the cold test planned for this week, although there's little point in proceeding until the turret is fixed. Completed mods/changes: 1) Grating turret - rotation balanced by adding wts in two places - worm spring force increased by ~ 3x 2) Slit wheel - axial spring force increased by ~ 4x - axial bearing 'dimpled' to decrease play 3) Detector bracket machined to move focus 2.0mm closer to spectrograph lens 4) Focus stage re-assembled with new ball drive and delrin gear for hard-stop 5) Electrical connector feed-through leak fixed and tested on lab test dewar 6) Changed bigdog, guidedog and littledog rack configurations to telescope congiguration 7) Checked new filter holder in guider filter wheel. (The filter holders are the only internal parts left to fabricate.) 8) Fabricated the final two window baffles 9) Completed all painting of internal parts 10) Cal box mirrors and lenses assembled. Waiting for mounting plate and lamp mounts from the shop. Warm tests: 1) Focus stage It still makes a racket but the vibration is less and there don't appear to be any problems. 2) Muxes Noisier than before. Probably related to animatics motors (see Peter's e-mail). Either we turn off the servos or insulate the feed-throughs. 3) Flexure tests (Saturday Nov 27) Specification From the SpeX PDR document, page 37: "The slit and grating turret positions must repeat and be stable to within one-tenth of the smallest slit width in the dispersion direction (0.2 pixels), and to within one pixel in the spatial direction (along the slit)." From page 39: "...flexure between the slit and the detector must be less than one-tenth the smallest slit width (2 pixels) when moving from zenith to 60 degrees over. Meeting this specification allows the use of calibrations done while the telescope is parked at zenith." I don't think we've got a hope of meeting the 0.2 pixel spec. Comments? A reasonable compromise is to do the flatfield and wavelength cals on the object or within 10 degrees in which case we can relax the spec to one pixel movement over 60 degrees. Slit wheel This was measured by imaging the slits on the guider mux. With the cryostat resting on the t-t window side it was tilted 60 degrees. This is the axis which saw most movement before the mod to the axial spring - 4 pixels in the dispersion direction none in the spatial direction. This time the measured movement was 0.5-1 pixel in the dispersion direction and none in the spatial. This meets the new specification (it is also the worst case). I then rotated the cryostat onto the motor-side and tilted 60 degrees in that axis and no saw no detectable movement. If this doesn't get worse during the cold test then we're OK. Grating turret Even before doing the tests I could feel play in the mechanism while installing the gratings. The play is in the direction of tilting the turret by pressing down on it, not in the rotation direction. Flexure was measured by imaging the lamp-line spectrum on the spectrograph mux. With the cryostat resting on the t-t window side it was tilted 60 degrees. Previously I measured about 2 pixels of movement in both the dispersion and spatial directions. This time I measured 12 pixels in dispersion and 2 in spatial. Much worse. This movement was the same for other grating/prism modes. I'm not sure why it got worse. Perhaps the 4-pixel movement of the slit masked the movement before? I did this test with the lid-roller on and off and it didn't make much difference! With the cryostat tilted 60 degrees resting on the motor side I measured about 2 pixels of movement in both dispersion and spatial. This is the axis in which balancing and increased spring force should help and indeed the movement is reduced from about 8 pixels to 2 pixels in the spatial direction. Once we establish that the flexure is not just an assembly problem, Vern and Doug need to get their heads together and decide whether to have one more go at modifying the mechanism or perhaps redesign the whole thing. John